The present invention relates to pulsed droplet deposition apparatus, for example drop-on-demand-ink jet printing apparatus and, in the most important example, provides voltage waveforms for the control of such apparatus.
Ink jet printing apparatus having a multiplicity of closely spaced parallel ink channels and channel separating piezo-electrically displaceable wall actuators have been disclosed for example in U.S. Pat. No. 4,879,568 (EP-B-0277703) and in U.S. Pat. No. 4,887,100 (EP-B-0278590). In such apparatus each channel is actuable by one or both of the displaceable side-walls. In a typical arrangement an external connection is provided which relates to each channel and when a voltage difference is applied between the electrode corresponding to one channel and the electrodes of the neighbouring channels, the walls adjacent to the channel are displaced causing the volume of the centre channel, depending on the voltage sign, to expand or to contract and an ink drop to be ejected from the nozzle communicating with the channel.
One feature of the above printing apparatus having displaceable side-walls is that operation of every channel at the same time is excluded. Operation takes place by dividing the printhead into two groups of odd and even channels, which are operated alternately. Alternatively the printhead is divided into groups of three, four or more channels which are operated in rotation (EP-A-0376532).
One waveform commonly used in the prior art is described in U.S. Pat. No. 4,161,670 in relation to the actuation of tubular ink jet actuating elements. In this case the applied voltage acts first to expand the diameter of the tubular drive element which contains ink, maintaining the expanded state for a period to admit ink into the ink tube, and then applying a voltage of reverse polarity to change the diameter of the tubular drive element from an expanded to a contracted state to eject an ink drop.
Such a waveform is implemented in the prior art by an oscillatory circuit, or if a pulsed waveform generator is employed pulses of both positive and negative polarity are required to generate it. Under conditions where drop-on-demand printheads are a mass produced component, the drive circuit necessarily takes the form of an integrated circuit chip, and such devices have the disadvantage of being considerably more expensive if required to handle bipolar signals.
Another disadvantage of the above waveform is that following drop ejection, there remains in the tubular actuator residual acoustic waves, and it is necessary to wait until these acoustic waves are damped before a further drop is ejected. This problem has been recognised in U.S. Pat. No. 4,743,924 and in U.S. Pat. No. 4,752,790 and in the former case it is proposed to provide an additional pulse to suppress acoustic reflection waves of the expulsion pressure wave at a time four periods L/c following the pressure wave (i.e. two characteristic times Tc).